When shooting in dim light conditions or at night, one may intend to shoot a long exposure to capture the beautiful streaks of lights from the traffic, or render the flowing wafer surface silky smooth. One way to achieve the long exposure is to shoot at small exposure such as f/22. The small aperture also helps to achieve large depth of field (DOF). If you use auto exposure, the correct shutter speed may exceed the camera’s longest exposure limit (30 seconds on many cameras). To obtain accurate exposure, you will need to meter at a wider aperture such as f/2.8 to ensure the shutter speed is within the camera’s specified limit. Once the correct exposure is determined, you can then stop down the lens and increase the exposure time accordingly to compensate. [Inspired by this discussion.]